My Japanese willow planted in the garden has thrived for about 10 years, but it lost its leaves early in the autumn and has very few this spring. I am very fond of the tree and would really appreciate some advice please. It is in clay type soil and the only tree there.
When you say your tree has a "few leaves" - are these leaves only on a few branches or are they dispersed throughout the tree? Do the leaves look normal? Are there any signs of pests or diseases? Did you fertilize your tree this spring? If not, that is something you might want to consider. The best thing you can really do at this time is follow a regular watering, feeding and pruning schedule to ensure your tree remains as healthy as possible.
On the branches that don't have leaves you may want to conduct a scratch test to see if there is any sign of life. Guidelines on how to conduct a scratch test can be found in the following article: http://www.starkbros.com/growing-guide/article/how-to-do-a-scratch-test/ With a scratch test, "green is good" and means the branch in question is still alive.
Below are some articles that will help you which discuss leafing out problems on trees and how to diagnose them: